This invention relates generally to power supplies and in particular to a voltage multiplier using a series combination of capacitors sequentially charged through an associated switching circuit.
Generating high voltages from lower voltage power sources is a common design problem in electronics applications. Many applications involve the use of display technology, such as cathode ray tubes, electroluminecscent (EL) displays, and cold cathode displays, that require voltage potentials in the hundreds or thousands of volts but at low current. A typical EL display is designed to operate with an a.c. (alternating current) voltage with a frequency of 400 hertz and 100 V rms potential but with less than 10 milliamperes of current draw for a predetermined level of brightness. EL display brightness can be adjusted by varying either the frequency or the voltage. Generating such a high voltage from the typical d.c. low voltages available for powering semiconductor components has traditionally been the function of an inverter-type power supply ("inverter"). Inverters are designed to convert a d.c. (direct current) voltage level to a different voltage level, usually a higher voltage, using active switching to convert the d.c. voltage to a pulsed voltage which in turn is applied to the primary winding of a transformer. Energy is stored in a magnetic field generated by the primary winding and a voltage is induced in the secondary winding. The secondary voltage induced depends on the turns ratio of the primary to secondary windings and the mutual coupling of the windings in a manner well known in the art. Inverters, typically simple circuits with high conversion efficiency, are available as self contained components with the input and output parameters specified so that the designer may select the particular device that best suits the application.
Inverters suffer from a disadvantage in that "noise" in the form of high frequency energy is created by the voltage conversion process. Noise from the transformer is in the form of magnetic flux which induces sway voltages in adjacent circuits. To mitigate this effect, inverter transformers are typically sheathed in ferromagnetic material to contain the magnetic flux. Furthermore, noise may also be coupled directly back into the low voltage power supply by the nature of the chopping or inverting action. When the d.c. voltage is switched, current spikes are created that require the addition of input filter circuits to filter it. If sensitive circuits are employed nearby, more elaborate filtering and isolating schemes are required to adequately isolate the noise of the inverter. Finally, the voltage and frequency of inverters are not easily varied, making the task of adjusting EL display brightness more difficult.
Passive voltage multipliers using capacitors to store energy have been employed in lieu of inverters of a true polarity reversing a.c. voltage can be obtained. Voltage multipliers have been constructed of series combinations of diodes and capacitors in a manner well known in the art. An alternating polarity sine wave signal allows the diodes, which conduct in only one polarity, to be utilized as switches for alternatively charging capacitors and then dumping the accumulated charge to other capacitors disposed in series. In this manner, a series combination of N capacitors may be charged to yield a total potential across the series combination of N times the charging voltage. The advantage of voltage multiplier circuit is that the energy storage during the voltage conversion process is performed with capacitors which store energy as accumulated charge rather than in a magnetic field. A voltage multiplier circuit, while providing a simple method of multiplying voltage, requires an alternating polarity input voltage to operate, a limitation that makes this topology impractical in applications where only a unipolar power supply is available, such as in battery-operated portable equipment, because of the difficulty of generating an alternating polarity voltage. Furthermore, this technique yields a high voltage that is d.c., rather than the a.c. potential required for EL displays, thus requiring additional switching circuitry.
In U.S. Patent application Ser. No. 08/311,470, filed Sep. 23, 1994, entitled "Voltage Multiplier Using Switched Capacitance Technique, and assigned to Fluke Corporation, a voltage multiplier is provided using a series combination of capacitors and an associated switching circuit which charges each capacitor in the series by momentarily connecting each capacitor to a d.c. input voltage in a sequential fashion. The d.c. input voltage of N volts is converted to an a.c. output voltage with a peak-to-peak value of V.multidot.(N+1) where V is the input d.c. voltage and N is the number of capacitors in the series combination. To develop an output signal of 240 volts peak-peak from an input voltage of 30 volts d.c., seven capacitors would thereby be required in the series combination, along with associated charging and switching circuitry. It would be desirable to provide a voltage multiplier using a switched capacitance technique to maintain simplicity and relative noise advantages over other techniques but employing fewer components, thereby reducing manufacturing cost and circuit board space.